This invention relates generally to a method and extrusion die for forming a tendon suitable for various applications, including post tensioning concrete. More particularly, this invention relates to a method and die arrangement for forming a post tensioning tendon which includes a central, multiple-wire strand coated with a corrosion inhibitor and then encased within a plastic jacket which includes a seam.
Various attempts have been made toward the manufacture of a cost-effective post tensioning tendon which includes a multiple-wire strand and is capable of good handling characteristics in the field. Probably one of the earliest attempts was a paper-wrapped tendon. That tendon included a seven-wire strand coated with a corrosion inhibitor and then spirally wrapped with some type of paper, usually a reinforced paper. However, this paper-wrapped tendon exhibited various shortcomings. For example, the paper would develop tears in the jacket, resulting in the leakage of the corrosion inhibitor which could develop unsightly stains on concrete. Also, when the paper-wrapped tendon was placed in the forms, the weight of the concrete could force the paper into the interstices of the multiple-wire strand, creating a mechanical interlock inside the tendon which reduced the effectiveness of the tendon during tensioning of the strand for the reinforcement of the concrete.
In response to the problems associated with the paper-wrapped tendon, other tendons were developed, including the push-through tendon and the lap-seam or cigarette-wrapped tendon. Again, both of these tendons included disadvantages.
The push-through tendon was made in a process which included the application of a corrosion inhibitor to the strand and then inserting the coated strand into a previously manufactured plastic tube. By its very nature, this manner of production was slow and therefore resulted in relatively high manufacture and labor costs. Also, an air gap typically resulted in the push-through tendon as a result of the method of manufacture, because of the desirability to make the internal dimension of the plastic tube greater than the outer diameter of the coated strand in order to facilitate the insertion of the strand into the tube. This necessarily resulted in an air gap within the tendon. That air gap may permit the intrusion of moisture into the tendon that can result in long-term corrosion.
The lap-seam tendon was made in a method where a corrosion inhibitor is applied to a multiple-wire strand and then a previously-formed strip of plastic material is curled around the coated strand. As the plastic material is curled around the strand, heat is applied to the overlapping edges of the strip and then the overlapping edges are pressed together to form a seam. This process of manufacture also results in relatively high production costs. Additionally, this product had the tendency to develop splits along the seam, again permitting the intrusion of moisture for long term corrosive effects. Further, because an air gap typically exists within the jacket of the lap-seam tendon, this tendon exhibited undesirable handling characteristics in the field, because the coated strand tended to slip inside the jacket or the jacket could inadvertently and somewhat easily be slipped off the strand in the field at times.
One attempt to overcome the problems of the paper-wrapped, the push-through, and lap-seam tendons is set forth in U.S. Pat. No. 3,646,748. In general, this patent discloses a post tensioning tendon which is made in a process by extruding a seamless, plastic, tubular jacket around a coated strand. Because the method of this patent can be conducted in a continuous process at relatively high speeds, manufacturing costs can be reduced. Also, the jacket formed in this process is tightly in engagement with the coated strand, eliminating the undesirable handling characteristics associated with the lap-seam and push-through tendon with respect to the jacket inadvertently slipping off the strand because of the air gap within the tendon. However, substantial license fees are required for practicing the process of making the product covered by U.S. Pat. No. 3,646,748.